A need frequently arises for the sealing of a container, e.g. a container of plastic material, by a laminate whose shape has been adapted to the shape of the opening, prior to the placing of the laminate on, or into, the opening. It has been known to seal containers with the aid of a foil of multilayer material, where the foil, after the container has been sealed, is trimmed in order to remove the parts of the foil projecting outside the edge of the container opening.
One problem which is encountered on the sealing of containers by means of foils, and especially by means of multilayer foils, is that on heating the foil it tends to become deformed because of curling. Since the foil as a rule is relatively thin, it assumes ambient temperature after a short time or is heated rapidly on being subjected to thermal radiaton. Such an environment exists for the foil when containers are to be sealed which are filled with warm or hot contents, and generally occurs on packaging foodstuffs. In certain applications, and especially when the foil is composed of layers of materials having different coefficients of linear expansion, the problem associated with the deformation of the foil is accentuated. When the foil gets warm it will curl so that if on the foil, e.g., the layer with the greatest coefficient of linear expansion faces the container opening, the circumferential outer edge of the foil will be raised, whereas on opposite placing of the layer the outer edge is displaced downwards.
The aforementioned deformations, as a rule, can be overcome if a foil in the form of a band is stretched over one or more containers and is fixed to the opening part of the respective container, so that the foil during sealing is kept stretched by mechanical means which counteract the tendency of the foil to curl. When the foil has thus been fixed, any projecting parts are cut off the respective container. However, in cases where the foil consisting of several layers is cut into wafers, each of a shape corresponding to the shape of the opening part of the container, prior to being fixed to the opening part of the container, it is not possible to stretch the foil by mechanical means, e.g. between its edges, so as to prevent the undesirable deformation (curling up) which on sealing of e.g. hot-filled containers entails only too often the occurrence of leaky closures.